Sculpting Performance Workflow
CGBoost has some good vids on the topic. Also, try the sculptmode features branch. This has an excellent volumetric remesher that can convert your dyntopo object to quads for you to use the faster multires modifier with. This branch of Blender has lots of improvements over the ordinary Blender 2.8, but it's not official so be warned there may be bugs. Back up files, keep the old Blender around when you update just in case, etc., etc. 
You could use the free and open source tool instant meshes for quad remeshing, too.

It's generally bad practice to have uneven detail density on a sculpture- you should get the low-frequency details right before the high-frequency details. But anyways, in your situation it's probably well advised to cut the mesh into pieces, then fill the holes, sculpt, and use Bool Tools to combine them later.

Filling the holes is pretty easy, enter edit mode once you've separated a part and go to mesh>cleanup>fill holes (or search with F3), and set the 'number of sides' to the max (100). If it fails to fill a hole (too many sides), try using select>select by trait> non-manifold, (in edge select mode, it won't show up if you're selecting faces) but be warned that it might select more than just the edges you want to fill. Once you have removed any unnecessary edges from the selection, press f to fill. I usually ctrl-f>Poke Face and then do some dyntopo sculpts on it to even out the triangles. Then you can use remesh or instant meshes to turn each part into quads, and multires.